Pruzinská Adriana, Tanner Gaby, Anders Iwona, Roca Maria, Hörtensteiner Stefan
Institute of Plant Sciences, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland.
Proc Natl Acad Sci U S A. 2003 Dec 9;100(25):15259-64. doi: 10.1073/pnas.2036571100. Epub 2003 Dec 1.
Chlorophyll (chl) breakdown during senescence is an integral part of plant development and leads to the accumulation of colorless catabolites. The loss of green pigment is due to an oxygenolytic opening of the porphyrin macrocycle of pheophorbide (pheide) a followed by a reduction to yield a fluorescent chl catabolite. This step is comprised of the interaction of two enzymes, pheide a oxygenase (PaO) and red chl catabolite reductase. PaO activity is found only during senescence, hence PaO seems to be a key regulator of chl catabolism. Whereas red chl catabolite reductase has been cloned, the nature of PaO has remained elusive. Here we report on the identification of the PaO gene of Arabidopsis thaliana (AtPaO). AtPaO is a Rieske-type iron-sulfur cluster-containing enzyme that is identical to Arabidopsis accelerated cell death 1 and homologous to lethal leaf spot 1 (LLS1) of maize. Biochemical properties of recombinant AtPaO were identical to PaO isolated from a natural source. Production of fluorescent chl catabolite-1 required ferredoxin as an electron source and both substrates, pheide a and molecular oxygen. By using a maize lls1 mutant, the in vivo function of PaO, i.e., degradation of pheide a during senescence, could be confirmed. Thus, lls1 leaves stayed green during dark incubation and accumulated pheide a that caused a light-dependent lesion mimic phenotype. Whereas proteins were degraded similarly in wild type and lls1, a chl-binding protein was selectively retained in the mutant. PaO expression correlated positively with senescence, but the enzyme appeared to be post-translationally regulated as well.
叶绿素（chl）在衰老过程中的分解是植物发育不可或缺的一部分，并导致无色分解代谢产物的积累。绿色色素的丧失是由于脱镁叶绿酸（pheide）a的卟啉大环发生氧化裂解，随后还原生成一种荧光性的chl分解代谢产物。这一步骤由两种酶，即脱镁叶绿酸a加氧酶（PaO）和红色叶绿素分解代谢产物还原酶相互作用组成。PaO活性仅在衰老过程中被发现，因此PaO似乎是chl分解代谢的关键调节因子。虽然红色叶绿素分解代谢产物还原酶已被克隆，但PaO的本质仍然难以捉摸。在此，我们报告了拟南芥（AtPaO）的PaO基因的鉴定。AtPaO是一种含Rieske型铁硫簇的酶，与拟南芥加速细胞死亡1相同，且与玉米的致死叶斑1（LLS1）同源。重组AtPaO的生化特性与从天然来源分离的PaO相同。荧光性chl分解代谢产物-1的产生需要铁氧还蛋白作为电子源以及脱镁叶绿酸a和分子氧这两种底物。通过使用玉米lls1突变体，PaO在体内的功能，即在衰老过程中脱镁叶绿酸a的降解，得以证实。因此，lls1叶片在黑暗培养期间保持绿色，并积累了导致光依赖性类病斑表型的脱镁叶绿酸a。虽然野生型和lls1中的蛋白质降解情况相似，但一种chl结合蛋白在突变体中被选择性保留。PaO表达与衰老呈正相关，但该酶似乎也受到翻译后调控。
